Add top-level namespace to Usage Overview panel

What does this MR do and why?

Adds a new element with the top-level namespace's full name, type, visibility level icon and avatar to the Usage Overview panel on the Value Stream Dashboard.

MR acceptance checklist

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Before	After

How to set up and validate locally

1. Please make sure you have a GitLab Ultimate license.
2. Feature.enable(:group_analytics_dashboard_dynamic_vsd)
3. Create a new (or choose an existing) top-level group and invoke the seed script: GROUP_ID=<group-id> FILTER=vsd_overview_counts SEED_VSD_COUNTS=1 bundle exec rake db:seed_fu
4. Navigate to the group above and, from the left sidebar, click the Analyze > Analytics dashboards link
5. The group dashboard list should appear, select Value Stream Dashboard
6. Verify that the top-level namespace and related elements appear in the Usage overview panel.

Related to #439699 (closed)